I dont entirely know but I can take a stab - I think it's likely that either that node's owner pulled your node's public key and started using it (probably not), or, more likely, these tiny devices dont have a lot of entropy to work with when generating keys leading to less randomness and more collisions than should be likely for the algorithm and key size. Basically, you should probably regen your keys to make sure, as that node may have the same keys by accident. Make sure to tell people you know on the network to delete you as key changes are not handled well and will probably lead to broken (or at least broken looking) dms until your node is deleted and rediscovered.